Forbidden? Says who? Just another typical instructor's personal opinion and there is no shortage of those as we can see in this thread.
A37whatever,
It was also the view a mob called Qantas Airways Ltd., as expressed in the "opinion" of Qantas Airways Ltd Operations Manuals, but Hey!!! what would Qantas performance engineers know about operating aircraft.
In fact, in the aircraft used in Qantas cadet pilot training (mostly Aztec) and after a rigorous risk analysis, Qantas went further, and required that, once the gear was down and the flaps out to any setting, even on all engines a landing was required.
The basis of the risk analysis was quite simple, an all engines approach could turn into an engine out approach at any time.
As far as I am concerned, that is the way to go in any light twin that I have flown (or any B707 or 747 on two) and that includes DC-3/C-47, even though all DC-3/C-47 have dual hydraulic pumps.
That is also the point where you should forget "blue line speed" and fly a speed based on a Vref, with suitable but small increments wind condition and gusts.
FAR 23 light twins should be treated for what they are (and how they have been certified), a single engine aircraft, with twice the chance of engine failure, all the "second" engine does is extend the glide.
What amazes me these days is the loss of "aviation knowledge", go back 40 years (when small twins started to arrive), and what I have said above was common and accepted knowledge, there was no "debate" because the performance limitations were obvious fact, NOT "opinion".
Tootle pip!!